Correlation in inbred human populations between clinical reaction to vaccine, serum antibody titer to measles hemagglutinin and titer to pneumococcal polysaccharides on the one hand, and kinship and HLA and immunoglobulin haplotypes on the other hand, will be studied: to determine the generality of the associations with respect to various inbred human populatoins, various antigens and various other genetic markers; to partition the total genetically controlled effect between effects associated with markers in different linkage groups and interactive effects; to measure the extent to which the effects are expressed in additive or dominant manner, by helper or repressor functions; and to map the controlling genes relative to markers such as individual HLA genes. An ancillary project involves collection of more data on the deficit of homozygous persons in these populations. The data will be used to determine the age at which the deficit appears and thus to develop hypotheses concerning the forces responsible for the loss of homozygous individuals and themechanism behnind it.